DK159437B - PROCEDURE FOR CONTINUOUS EXTRACTION OF PROCESS GAS - Google Patents

Abstract

Process for the continuous separation of maleic acid anhydride from process gases from the catalytic oxidation of hydrocarbons, e.g., n-butane, in the steam phase. The process gases are treated with a solvent, which is a cycloaliphatic acid ester. Typically, the cycloaliphatic acid ester is a dialkyl ester, having 4 to 8 carbon atoms in each alkyl group, of hexahydrophthalic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid or methylhexahydrophthalic acid.

Description

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The invention relates to a particular process for the continuous extraction of maleic anhydride from process gases.

It is known to produce maleic anhydride by catalytic oxidation in the vapor phase of benzene, occasionally also n-butane, butene and other C According to known methods, the maleic anhydride is recovered from the process gases in the manner described below.

First, the gases are cooled to temperatures of 55--60 ° C. This temperature is below the so-called dew point of maleic anhydride 10, but still above the dew point of water, which is, for its part, also a by-product of the reaction. Thus, about 50-60% of the maleic anhydride can be condensed from the process gases. The lower the condensation temperature is chosen, the more the water contained in the gas is bound by the condensing maleic anhydride to form maleic acid. However, the formation of maleic acid is undesirable as it is a solid product which is highly soluble in molten maleic anhydride. This leads to constipation in the apparatus, resulting in 20 capacity and yield losses.

The non-condensed maleic anhydride is recovered as maleic acid by washing the gas stream with water. The malic acid must then again be converted into maleic anhydride by dehydration. In isomerization processes, fumaric acid can then also be formed, which eventually leads to large yield losses.

In U.S. Patent No. 3,891,680, the washing operation is carried out in esters of phthalic acid with C ^-Cg alcohols. According to DE-OS No. 2,444,824, dibenzylbenzene is used as a washing liquid. Finally, GB-PS No. 1,443,411 discloses polymethyl benzophenones scm washing liquid. However, these systems exhibit serious deficiencies such as lack of stability of the wash liquid, especially the phthalic acid ester, leading to additional costs of replacing the lost wash liquid, the high prices of the wash liquid using polymethylbenzophenone, and finally, that both

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2 polymethylbenzophenones and dibenzylbenzenes are solid at room temperature.

It is therefore the object of the present invention to provide a process for the continuous extraction of maleic anhydride which does not have the disadvantages mentioned.

In order to fulfill this object, the invention relates to a process for the continuous extraction of maleic anhydride from process gases from the catalytic oxidation of vapor phase hydrocarbons by treating the process gases 10 with a solvent, which process is characterized by using as a solvent a dialkyl ester of 4-8 carbon atoms in each alkyl group of hexahydrophthalic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid or methylhexahydrophthalic acid.

The use of the present solvents is particularly advantageous when reaction gases from the catalytic oxidation of n-butane in the vapor phase are used as process gas. In this case, the water concentration in the process gases is much greater than e.g. by 20 a process using benzene. Thus, by oxidation of n-butane, conditional on the high water content, it would only be possible by partial condensation of 30-35% maleic anhydride separation. The remaining 65-70% must be washed out with water as solvent and appear as 25 maleic acid. During high energy consumption and loss of yield, the maleic acid must be separated from the water and dehydrogenated to maleic anhydride.

The process of the present invention is carried out in apparatus known per se.

FIG. 1 and 2 of the drawing schematically show the apparatus used in the examples below and are explained in detail therein.

The process gases from the reaction can first be subjected to partial condensation. Partially condensed or non-condensed process gases are fed into at least one wash column.

It can for example. be a bottom column or a column filled

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3 with Raschig rings.

The process gas is fed into a first column. The solvent flows toward the gas stream and is increasingly charged with the maleic anhydride. The gas flow temperature 5 and maleic anhydride concentration thereby decrease, whereas the solvent is increasingly enriched with maleic acid.

Similarly, additional columns can be linked to the first column. The limit of adsorption of the maleic anhydride is set at the dew point of the water in the reaction gases.

It is possible to wash more than 98% of the maleic anhydride out of the process gases without significant maleic acid formation.

Continuously or, where appropriate, incrementally, the mixture containing 10-30% maleic anhydride in the solvent can be led to a fractional distillation. The maleic anhydride is distilled off and the high boiling solvents can again be circulated to the adsorption column.

20 The solvents in question have a number of advantages. They are liquid at room temperature and their viscosity is relatively low. Eg. are the known polymethylbenzophenones or dibenzylbenzenes fixed at room temperature. The solvents of this invention have high boiling points. Therefore, at the adsorption temperatures of the maleic anhydride from the process gases, the solvents exhibit only low vapor pressure. In gas saturation, only insignificant small amounts of solvent are lost.

3 ° The solvents in question also have a high chemical stability. After adsorption, they can be separated from the maleic anhydride by distillation and returned to the adsorption process. With the phthalic acid ester which e.g. is known from U.S. Patent No. 3,891,680, 35 always incur losses that must be replaced. Finally, the affinity of these solvents for water is insignificantly small. The formation of maleic acid is therefore reduced to a negligible extent.

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Example 1 100 kg of exhaust gas leaving the catalytic conversion of n-butane to maleic anhydride, also containing 2.65 kg of maleic anhydride and 4.8 kg of water, is fed to an absorption system consisting of two columns as depicted in the schematic flow diagram ( Figure 1).

The gases (line φ) enter a first column of absorption (Cl) at a temperature of 80 ° C. IN

column C, the gases are washed countercurrently with dibutyl hexahydrophthalate at a temperature of 80 ° C. The gases leaving column C are cooled to 60 ° C (by means of a heat exchanger not shown in Fig. 1) and enters 15 (conduit (2)), an absorption column C2 having a temperature of 60 ° C. In column C2, the gases are once again subjected to a washing operation with di-butyl hexahydrophthalate to such an extent that when they leave this column (line ©) in, their maleic anhydride content is only 0.06 kg.

The total amount of maleic anhydride absorbed is 2.59 kg, with an absorption yield of 97.7%.

16.53 kg of solvent, containing 1.1% by weight of maleic anhydride, is passed to the top of column C2 (line ©). At the end of column C2, there is 17.11 kg of maleic anhydride di-butyl hexahydrophthalate solution containing 4.4% maleic anhydride (line φ). This solution preheated to 80 ° C (by means of a heat exchanger not shown in Fig. 1) is introduced into the top of column C1.

At the outlet of column C1 (line φ), there is 19.12 kg of maleic anhydride dibutyl hexahydrophthalate solution containing 14.5 wt% maleic anhydride.

In column C, the absorption of maleic anhydride is 2.01 kg (77.6% of the absorbed maleic anhydride) and in column C2 the absorption of maleic anhydride is 0.58 kg (22.4% of the absorbed maleic anhydride). ).

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There are 6 theoretical absorption steps (three of which are in column C1 and the remaining three in column C2).

The 14.5% maleic anhydride solution is sent to a fractional distillation system (not shown in Fig. 1), thus obtaining as a top product practically pure maleic anhydride (2.59 kg) and as a base product dibutyl hexahydrophthalate (16, 49 kg) containing 1.1% by weight of maleic anhydride.

The dibutyl hexahydrophthalate formed is recirculated to the maleic anhydride absorption system via line 0.

Example 2 101.54 kg of exhaust gas (see Figure 2) leaving the catalytic conversion of benzene to maleic anhydride (line φ), also containing 2.96 kg of maleic anhydride and 2.82 kg of water, is fed to a partial condensation system which while cooling the gaseous mixture to 58 ° C, condensation of 1.54 kg of maleic anhydride (line φ) results. The non-condensed gas (100 kg containing 1.42 kg maleic anhydride) is fed (line (3)) to the absorption column C1 and washed countercurrently with di-isobutyl tetrahydrophthalate at a temperature of 58 ° C.

The gases leaving the column (line 0) contain only 0.07 kg of maleic anhydride. The remaining 1.35 kg of maleic anhydride has been absorbed into diisobutyl tetrahydrophthalate. The absorption yield is 95.1%. Therefore, given that 1.54 kg of maleic anhydride produced directly by partial condensation, the total yield of maleic anhydride is 97.6%.

There are four theoretical absorption steps in column 35 Cl. 8.25 kg maleic anhydride di-isobutyl tetrahydrophthalate solution containing 1.1% by weight of maleic II

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6 acid anhydride, lead to the top of the column (line Θ).

The solution / which is rich in maleic anhydride (line (D) containing 15% by weight maleic anhydride, 5 is of 9.60 kg. It is subjected to separation by fractionated distillation (not shown in Figure 2). The resulting product consists of 1). 35 kg of nearly pure maleic anhydride as the top fraction and 8.26 kg of maleic anhydride - di-isobutyl-tetrahydrophthalate with 1.1% by weight of maleic anhydride as the bottom fraction.

The latter solution is returned to column C1 via line (!) For recycling, whereas the nearly pure maleic anhydride produced as the main fraction is added to the maleic anhydride obtained by partial condensation (line φ) and distilled again to provide of practically pure maleic anhydride.

EXAMPLE 3 100 kg of exhaust gas leaving the catalytic conversion of isomeric n-butenes to maleic anhydride, also containing 3.06 kg of maleic anhydride and 4.97 kg of water, is sent to an absorption system consisting of two columns as shown in the schematic flow chart (Fig. 1).

The temperature of the gas entering (conduit (J)) the first absorption column C1 is 75 ° C.

In column C, the gas is subjected to a countercurrent wash with 30 di-isobutyl-hexahydrophthalate at a temperature of 75 ° C.

The gases leaving column C1 are cooled to 65 ° C (by means of a heat exchanger not shown in Fig. 1) and enter (conduction φ) absorption column C2 operating at 65 ° C. In column C2 35, the gases are finally washed with di-isobutyl hexahydrophthalate, so that the gases leaving said column only

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7 contains 0.06 kg of maleic anhydride.

The total amount of maleic anhydride absorbed is 3.00 kg with an absorption yield of 98%.

The amount of solvent conducted to the top of 5 column C2 (line @) amounts to 28.42 kg with 0.5 wt% maleic anhydride content.

At the end of column C2, there is 28.79 kg of maleic anhydride di-isobutyl hexahydrophthalate solution containing 1.77% by weight maleic anhydride (line ©).

This solution preheated to 75 ° C (by means of a heat exchanger not shown in Fig. 1) is fed into the top of column C1.

At the end of column C1 (line 0), there is 31.42 kg of maleic anhydride di-isobutyl hexahydrophthalate solution with 10 wt% maleic anhydride content.

In column C, 2.63 kg of maleic anhydride (87.7% of the maleic anhydride absorbed) is absorbed.

In column C2, the maleic anhydride absorbed amounts to 0.37 kg (12.3% of the maleic anhydride absorbed).

There are four theoretical absorption steps (two of which are in column C1 and two in column C2).

The solution containing 10% maleic anhydride is sent to a fractional distillation system (not shown in Figure 1) to give as a top product (3.00 kg) 25 virtually pure maleic anhydride and as a base product diisobutyl hexahydrophthalate (28.40 kg) with a content of 0.5% by weight maleic anhydride.

The di-isobutyl hexahydrophthalate produced is sent back to maleic anhydride absorption via 30 wire (£).

Example 4 100.74 kg (see Fig. 2) leaving the catalytic oxidation reaction of n-butane to maleic anhydride, also containing 2.66 kg maleic anhydride

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and 4.82 kg of water are passed (line φ) to a partial condensation system operating at a temperature of 64 ° C. Thus, it is possible to recover (wire ®) 0.74 kg of maleic anhydride (about 28% of the maleic anhydride produced in the reaction phase).

The non-condensed gases (100 kg containing 1.92 kg of maleic anhydride) are fed (line)) to the absorption column C1 and washed therein countercurrent with di-hexylmethylhexahydrophthalate at a temperature 10 of 64 ° C.

The gas leaving column C1 (line (§)) contains only 0.05 kg of maleic anhydride. Therefore, the absorption yield is 97.4%. The total yield of recovered maleic anhydride, taking into account that in the partial capacitor 0.74 kg of maleic anhydride (line φ) is recovered amounts to 98.1%.

There are three theoretical absorption steps in column C1. 17.72 kg of a mixture of maleic anhydride and di-hexylmethyl hexahydrophthalate containing 0.5% by weight of maleic anhydride are passed to the top of column C1 (line (D)).

The solution rich in maleic anhydride leaving the bottom of column C1 (line ©) / with a 10% maleic anhydride content is sent to a fractional distillation system (not shown in Fig. 2) to give as the top product 1, 87 kg of practically pure maleic anhydride and as a base product 17.72 kg of a mixture of maleic anhydride and di-hexylmethyl-hexahydrophthalate having a content of Q, 5% by weight of maleic anhydride.

The latter mixture is recycled to the top of column C1 (line ©), whereas the maleic anhydride produced as a top product by fractional distillation is combined with the maleic anhydride produced by partial condensation (line φ) and again purified by fractional distillation 5r5f >

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9 (not shown in Fig. 2) to provide practically pure maleic anhydride.

The inferior loss of wash liquid in the process 5 of the invention than in the method of US Patent No. 3,891,680 is evident from the following calculation: In Example 1, 16.53 kg of solvent having a maleic anhydride (MSA) content of 1.1% by weight are added. (corresponding to 16.35 kg of pure solvent) via line 4 at the top of column C2. At the outlet of column Cl (line 6), 19.12 kg of solvent with a content of 14.5 wt% MSA (corresponding to 16.35 kg of solvent) is recovered.

Thus, no loss of solvent occurs during gas washing. In the subsequent fractionated distillation 15, 16.49 kg of solvent containing 1.1% by weight of MSA (corresponding to 16.31 kg of pure solvent) is obtained as the base product.

Thus, a loss of solvent of 0.04 kg occurs.

In Example 2, 8.25 kg of solvent is used and 8.26 kg is recovered. The MSA content in both cases is 1.1%.

20 The apparent increase of 0.01 kg is attributable to measurement inaccuracies and rounding errors, and thus no measurable loss occurs.

In Example 3, 28.42 kg of solvent is used with 0.5 wt% MSA (28.28 kg of pure solvent) and 31.42 25 kg of solvent with 10 wt% MSA (also 28.28 kg of pure solvent) are obtained. There is thus no loss in gas washing. By analogous calculation of the 28.40 kg of base product, a content of 0.5% by weight of MSA is obtained 28.26 kg of pure solvent. This is equivalent to a loss of 0.02 kg.

Example 4 uses 17.72 kg of solvent and also 17.72 kg of solvent is recovered. In both cases, the MSA content is 0.5%. Also, no loss of solvent can be found here.

Thus, calculated on the amount of recovered MSA, there are 35 the following solvent losses:

Example 1: 0 / 1.5%

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Example 2: 0%

Example 3: 0 / 0.7%

Example 4: 0%.

The use of dibutyl phthalate in US Patent No. 3,891,680 shows a loss of 3%.

Thus, a clear technical advance is achieved by the method according to the invention in relation to this prior art.

Claims (2)

A process for the continuous extraction of maleic anhydride from process gases from the catalytic oxidation of vapor phase hydrocarbons by treating the process gases 5 with a solvent, characterized in that as a solvent, a dialkyl ester having 4-8 carbon atoms is used in each alkyl group of hexahydrophthalic acid, tetrahydrophthalic acid, methyl tetrahydrophthalic acid or methyl hexahydrophthalic acid. 10 Process according to claim 1, characterized in that the process gas is derived from the catalytic oxidation of n-butane in the vapor phase.